Molecular Biology of Valvular Heart Disease

164 Pages·2014·9.239 MB·English

by  Nalini M. Rajamannan (eds.)

Nalini M. Rajamannan
Editor
Molecular Biology of
Valvular Heart Disease
123
Molecular Biology of Valvular
Heart Disease

Nalini M. Rajamannan
Editor
Molecular Biology
of Valvular Heart
Disease
Editor
Nalini M. Rajamannan, MD
Division of Biochemistry and Molecular Biology
Mayo Clinic
Rochester, MN
USA
ISBN 978-1-4471-6349-7 ISBN 978-1-4471-6350-3 (eBook)
DOI 10.1007/978-1-4471-6350-3
Springer London Heidelberg New York Dordrecht
Library of Congress Control Number: 2014937313
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Dr. Harry and Concie Rajamannan, Lohini, Chapman, Hugh
and Hazen Mayo and My Catholic Faith
My Mentors and Colleagues, Dr. Thomas Spelsberg
and Dr. Christopher Johnson
Members of the NHLBI Working group
The co-authors of the Molecular Biology of Valvular
Heart Disease and Cardiac Valvular Medicine

Foreword
The history of cardiac valve cellular biology experiments began in the early
1980s with pioneering studies from the Mayo Clinic’s Department of Pediatrics
by Dr. Christopher Johnson. Dr. Johnson developed in vitro cell culture systems
to study the cellular biochemistry and molecular biology of the heart valve. The
development of these porcine cell culture systems took several years to isolate,
and clone the valvular sub-populations. It was clear from the start that there were
two morphologically distinct layers of cells resident in the valve tissue, an endo-
thelial cell layer and a subendothelial cell layer. These studies describing the
meticulous isolation of cardiac valve cells were reported in several seminal pub-
lications. Since then, Dr. Johnson’s cell culture techniques have been utilized in
valve biology laboratories around the globe and have provided the foundation for
understanding the cellular expression of extracellular matrix proteins both in vitro
and in vivo . His pioneering work determined the optimal technique for valve cell
isolation and identifi ed subtle changes in cellular morphology by light micros-
copy. These early studies also provided some initial biochemical and molecular
characterization of both endothelial and subendothelial cell populations.
In 1983, Johnson’s laboratory established culture lines of porcine endothe-
lial cells derived from the aortic valve. These cells had the typical morphology
of vascular endothelial cells in culture and contained angiotensin-converting
enzyme activity found in other endothelial cells. However, they differed from
endothelial cells derived from the ascending aorta in a unique way: aortic
valvular endothelial cells were defi cient in their ability to synthesize fi bro-
nectin. This fi nding was confi rmed i n vivo using immunohistochemical stain-
ing of tissues. This was an intriguing fi nd because fi bronectin is a major
biosynthetic product of other vascular endothelial cell types. Aortic valve
cells are relatively defi cient in this ability, a fi nding that carries signifi cant
implications for aortic valvular disease
In 1987, Johnson’s laboratory established culture lines derived from the
subendothelial region of the porcine aortic valve. These cells were isolated by
extensive collagenase digestion of valvular tissue and were serially propa-
gated i n vitro with stable morphology. In sparse culture by phase contrast
microscopy valve, subendothelial cells, later termed myofi broblasts, morpho-
logically resembled skin fi broblasts. When confl uent in the culture dish, how-
ever, the valve subendothelial cells assumed a morphology similar to that of
cultured vascular smooth muscle cells, in that they formed ridges and piles.
Subendothelial cells also formed calcifying nodules i n vitro , the fi rst descrip-
tion in the literature of this potentially signifi cant phenomenon.
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viii Foreword
In 1988, Rajamannan and Johnson reported cell culture experiments that
identifi ed potential cell to cell communication occurring between valve endo-
thelial cells and subendothelial myofi broblasts. The experiments examined
materials released into the culture supernatant by endothelial cells that would
affect growth and other behaviors of myofi broblast cells. This i n vitro culture
model evolved over the next 25 years to identify the role of the components
of this conditioned media, including isolation of mitogenic proteins, and to
characterize the expression of endothelial nitric oxide synthase in the aortic
valve endothelium. The cell-cell architecture assays set the foundation for the
evolving hypothesis that the endothelial cell, under conditions of oxidative
stress, is responsible for the initiation event of the myofi broblast cell to signal
extracellular matrix synthesis, which is then responsible for the calcifi cation
process in the valve. This cell-cell communication hypothesis has now
evolved into the concept of the stem cell niche in the aortic valve.
The fi eld evolved rapidly in the 1990s with the published fi ndings from
large databases, such as the Cardiovascular Health Study from the University
of Washington. These fi ndings demonstrated that the risk factors for valvular
heart disease are similar to those for vascular atherosclerosis, including the
traditional risk factors – lipids, hypertension, obesity, male gender. From 2000
to the present, i n vivo and i n vitro models have been described to determine the
cellular basis of the calcifi cation in the aortic valve. This included the appear-
ance of an osteoblast-like phenotype, the phenotype of the myxomatous
changes in the mitral valve, and a chondrogenic-like phenotype, as described
by investigators at the University of Pennsylvania, the Mayo Clinic, Rice
University, and Northwestern University. This work all began with the i n vitro
cell culture systems fi rst described by Johnson and Rajamannan 30 years ago.
Over the past decade, animal models have been developed to study the cel-
lular processes of the cardiac valve. These have included rabbits and genetic
knock-out mice using different concentrations of experimental hypercholes-
terolemia diets, different lengths of diets, and renal failure to determine the
critical fi ndings of apoptosis, cell proliferation, atherosclerosis, early mineral-
ization, chondrocyte hypertrophy, and fi nally calcifi cation. The specifi c cell
assays, both i n vivo and i n vitro , as well as e x vivo models, and techniques
presented in this textbook, Molecular Biology of Valvular Heart Disease, will
become a handbook for current and future valve biologists throughout the
world.
Rochester, MN , USA Nalini M. Rajamannan, MD, FACC, FAHA
Santa Fe , NM , USA Christopher M. Johnson, MD, MA, FAAP
Pref ace
The National Heart, Lung, and Blood Institute (NHLBI) convened a Working
Group of investigators on September 21, 2009, in Chicago, Illinois, to advise
the NHLBI on the state of the science and on new research directions to
improve the understanding, and ultimately, the treatment of calcifi c aortic
valve stenosis.
Calcifi c aortic valve disease (CAVD) covers a spectrum of disease from ini-
tial changes in the cell biology of the valve leafl ets, through early calcifi cation,
tissue remodeling and aortic sclerosis, to outfl ow obstruction and aortic stenosis.
The later stages are characterized by fi brotic thickening of the valve leafl ets and
the formation of new blood vessels and calcium nodules – often including the
formation of actual bone – throughout the valve leafl ets but concentrated near
the aortic surface. Although CAVD is more common with age, it is not an inevi-
table consequence of aging. CAVD appears to be an actively regulated disease
process that cannot be characterized simply as “senile” or “degenerative.”
E pidemiological studies show that some of the risk factors for CAVD are
similar to those for vascular atherosclerosis. Age, gender, and certain clinical
factors are all associated with an increased risk of CAVD. Clinical risk factors
associated with the presence of CAVD include elevated low-density lipopro-
tein (LDL) cholesterol, but the association is relatively weak in those over 65
years old, the group at greatest risk of progressing to aortic stenosis. Other
factors include smoking, hypertension, shorter height, lipoprotein (a) level,
metabolic syndrome, type II diabetes, end-stage renal disease (but not mild to
moderate renal disease), and imbalances in calcium or phosphate metabo-
lism. However, the factors associated with disease initiation may differ from
those that promote disease progression.
Although aortic stenosis may occur in individuals with otherwise anatomi-
cally normal aortic valves, congenital valve abnormalities markedly increase
the risk. Nearly half of the individuals with aortic stenosis have a bicuspid
aortic valve (BAV), an aortic valve that developed with two functional leafl ets
instead of the normal three. BAV occurs in about 0.6 % of the population and
is the most common congenital cardiac malformation. Although the causes of
BAV are unclear, genetic factors have been identifi ed in some cases. CAVD
tends to develop at an earlier age in individuals with BAV and to progress
more rapidly for reasons that are poorly understood. Genetic mutations asso-
ciated with BAV that cause cellular dysfunction may also predispose an
i ndividual to other congenital heart defects or to dilation and dissection of the
ascending aorta.
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